1. Field of the Invention
This invention relates to semiconductor memory devices and particularly to high density dynamic random access memory cells and methods for their manufacture in sub-micron technologies.
2. Description of the Prior Art
Designers of technologies for producing semi-conductor devices have been continually pressured to increase effective device densities in order to remain cost and performance competitive. As a result, VLSI and ULSI technologies have entered the sub-micron realm of structural dimensions and now are designing technologies in the deep submicron feature size range. In the foreseeable future absolute atomic physical limits will be reached in conventional two-dimensional design approach to semiconductor device design. Traditionally, Dynamic Random Access Memory (DRAM) designers have met the severest of challenges in advancing technologies by pushing the limits of feature size resolution with each generation of DRAM. For example, designers of 64 K bit DRAMs were perplexed to learn that a practical physical limit to charge capacity of storage capacitors for planar cell layouts had already been reached due to the minimum charge capacity required to allow reliable data signal sensing in the presence of naturally occurring atomic particle radiation inherently present in fabrication materials and the operating environment. Storage capacitors in the range of about 50 femtofarads were considered to be a physical limit. From a practical view, this limitation prevented a continuation of the scaling of DRAM dimensions and voltages initiated in the early 1980s. Reduction in the surface area of semiconductor substrate utilized by the DRAM storage capacitor has been severely restricted. Due to deceases in the thickness of reliable capacitor dielectric materials, existing 1 Megabit (1 Mb) DRAM technologies continue to enjoy the freedom of planar, two-dimensional device and circuit design. Beginning with 4 Mb DRAMs, the world of three-dimensional design has been utilized to the extent that the simple single device/capacitor memory cell has been altered to provide the capacitor in a vertical dimension. In such designs, the capacitor has been formed in a trench formed in the surface of the semiconductor substrate. In yet denser designs, other forms of three-dimensional capacitors have been proposed, such as stacking the plates of the capacitor above the transfer device. Such designs, however, present difficulties in forming the interconnections to the required word access and data bit lines to the DRAM memory cell. Additional designs have been proposed in which the transfer device and its associated capacitor are both formed within a trench of preferably minimum feature size. Currently, insurmountable processing difficulties make such designs impractical for product manufacturing processes.
A large number of proposals for 16 Mb and greater density DRAM cell designs have avoided continuing development of trench cell technology because of the existence of charge leakage mechanisms known to be present in trench capacitor structures. As these leakage mechanisms have become known, extensions of trench DRAM cells designs have been used successfully in 16 Mb designs.
The following references describe various aspects of prior art techniques used in DRAM and other semiconductor technologies.
The article "Trench and Compact Structures for DRAMs" by P. Chatterjee et al., International Electron Devices Meeting 1986, Technical Digest paper 6.1, pp. 128-131, describes variations in trench cell designs through 16 Mb-DRAM designs, including the Substrate Plate Trench (SPT) cell described in more detail in U.S. Pat. No. 4,688,063 issued Aug. 18, 1987 to Lu et al. and assigned to the assignee of the instant invention. The SPT cell uses a highly conductive substrate as the DRAM cell plate. The storage node of each cell is formed in a deep trench in the substrate. U.S. Pat. No. 4,801,988 issued Jan. 31, 1989 to Kenney and assigned to the assignee of the instant invention, describes an improved SPT cell which includes a thick isolation region formed within the trench to enable higher density packing of DRAM cells. The article "CMOS Semiconductor Memory Structural Modification to Allow Increased Memory Charge" anonymous, IBM Technical Disclosure Bulletin, Vol. 31, No. 11, April 1989, pp. 162-5, teaches a method of isolating the substrate plate of an SPT cell from support devices by providing a buried region under support devices in order to allow the plate reference voltage to be separately biased at an optimum Vdd/2 volts.
U.S. Pat. No. 4,912,054 issued Mar. 27, 1990 to Tomassetti describes methods of isolating bipolar-CMOS circuit devices through the use of various epitaxial layers as commonly found in bipolar device technologies. The article "A 45-ns 16-Mbit DRAM with Triple-Well Structure" by S. Fujii et al., IEEE Journal of Solid-State Circuits, Vol. 24, No. 5, October 1989, pp. 1170-1175, describes techniques for isolating various different functional device types in which the entire array of trench DRAM cells is formed within a surface implanted P-well.
U.S. Pat. No. 4,829,017 issued May 9, 1989 to Malhi describes a method of forming a buried doped layer in a substrate by forming a shallow trench, protecting its sidewalls, further extending the trench and finally doping the walls of the extended trench to form a continuous doped region useful as the storage node of a trench DRAM.
The article "New Well Structure for Deep Sub-micron CMOS/BiCMOS Using Thin Epitaxy over Buried Layer and Trench Isolation" by Y. Okazaki et al., 1990 Symposium on VLSI Technology, Digest of Technical Papers, paper 6C-4, pp. 83-4, describes the use of buried epitaxial layers to isolate surface devices from the substrate.
The following references relate specifically to variations in SPT DRAM cells in which a buried region of opposite conductivity type from the substrate is used a one plate of the DRAM storage capacitor. U.S. Pat. No. 4,918,502 issued Apr. 17, 1990 to Kaga et al. describes a buried plate trench DRAM cell in which the storage node of the cell and a sheath plate are formed in a single trench. At the bottom of the trench a diffusion of opposite type from the substrate is formed such that the diffusions of adjacent cells interconnect forming a grid-like structure. One or more trenches not associated with a DRAM cell is formed to act as a reach through to enable the doped region to be biased at a suitable reference voltage. FIG. 12, thereof, clearly illustrates the grid-like aspect of the buried region. European published application 0 283 964, published Sep. 28, 1988 describes a buried plate SPT DRAM cell in which an out-diffused region from the DRAM trenches, similar to that in Kaga et al., in which the diffused region forms the plate of the SPT cell. As in Kaga et al. a grid-like region is formed and is contacted by a non-cell trench. U.S. Pat. No. 4,873,560 issued Oct. 10, 1989 to Sunami et al, describes yet another buried plate SPT cell in which the access transistor is formed in the cell trench. FIG. 30, thereof, and its related text, describes the importance of maintaining the grid-like structure of the buried region in order to enable proper operation of the cell transfer device. Sunami et al, further cautions that in the event that opening in the grid-like buried region should be "filled by the depletion layer" isolating the surface devices from the substrate a separate connection can be made to the "isolated" surface region in order to bias it to the same potential as the substrate. UK Patent Application GB 2 215 913 A, published Sep. 27, 1989 describes yet another variation in the buried SPT DRAM cell design in which the dopant for the buried region is provided by ion implantation into the sidewalls of the deep trench of the DRAM cell. Finally, U.S. Pat. No. 4,794,434 issued Dec. 27, 1988 to Pelley, describes a buried plate SPT DRAM cell formed using bipolar device processing methods in which the buried plate region is formed from a buried sub-collector structure normally part of a bipolar transistor.
While the above cited references illustrate the diverse and concentrated efforts made by DRAM designers in attempting to overcome the inherent barriers in continuing to reduce the size, and increase the density, of DRAM cells, none provide the capability to carry DRAM technology into the sub-0.5 micron feature size range, a feat which must be achieved in order to continue the two decade "tradition" of providing ever increasing density of DRAM technology. DRAM designers have turned to the process-complicating use of "stacked capacitor" DRAM cells knowing that the addition of processing steps decreases the manufacturability of a specific design.
Referring to FIG. 1, there is shown a schematic cross-sectional view of the basic Substrate Plate Trench (SPT) DRAM cell described in U.S. Pat. No. 4,688,063 to Lu et al. entitled "Dynamic RAM Cell with MOS Trench Capacitor in CMOS". A P+ type semiconductor substrate 10 is provided with a lightly doped epitaxial surface layer 11 in which is provided N-type retrograde implanted well 12 formed at its upper surface in which the transfer device 14 is formed. A control gate electrode 16 is responsive to signals from word line circuitry to couple data signals applied to the bit or data line diffused region 18 to the diffused storage node region 20. A deep trench 22 is provided in which a storage capacitor is formed. A polysilicon storage node plate 24 is formed in the trench and isolated from the substrate 10 by a thin storage node dielectric, not shown. A conductive strap 26 connects the diffused node 20 to the plate 24.
Manufacturing experience has shown that the SPT DRAM cell described is not suitable for extension To greater than 16 Mbit applications due in part to the performance limitations of P-array transfer devices and the existence of a parasitic device formed by the diffused storage node 20, the polysilicon plate 24 and the substrate 10. Simple conversion to N-type transfer devices is not practical and reduction of electrical stress on the capacitor dielectric by using Vdd/2 reference node biasing is not possible. The subject invention addresses the unsolved problems of the prior art by providing a solution to barriers presented in extending the manufacturability of the simple SPT cell to 64 Mb DRAM and beyond.